It derives from the pagan belief that malevolent spirits inhabited wood, and that if you expressed a hope for the future you should touch, or knock on, wood to prevent the spirits from hearing and presumably preventing your hopes from coming true.

Hugh Morrison, London UK

From touching the wood of the cross.

Jeremy, Alumim Israel

"Touching wood" derives from pantheistic religions where trees, rivers etc were supposedly inhabited by spirits or deities. To touch a tree was to seek the spirit's blessing and/or ward off its wrath.

Brian Clapham, Cobham Surrey

There is no evidence to suggest that this phrase is anything other than the legacy of a children's game (such as tig) where 'touching wood', or being 'in den' or saying 'keys' prevented one from being caught. There is no evidence of it having been used before the 19th century - which would suggest no earlier pagan or Christian origins.

Amanda Fergusson, Kilmarnock Ayrshire

I've been informed it's actually "Touch wood and whistle" in full, i.e. touch a bit of wood, then a quick audible whistle. Still don't know where it's from though.

Spratt, Lewisham UK

The phrase originates from rural Hertfordshire where it is considered sexual innuendo...no explanation needed.

Nicholas, Cheshunt United Kingdom

Around here the reference is most certainly to grabbing the nearest penis, playfully.

Barrington Lomas, Leamington Spa, Warwickshire

The term Touch Wood actually comes from the 18th Century Auction House/ Barns. If you placed a bid on live stock or an item you would hope that the auctioner would "touch wood" at your bid. When the auctioner touched wood you would have won that lot. Hence the saying " I hope I win "Touch Wood" If you did win the auctioneers hammer "gavel" which is made of wood would also fall and hit the block. Touch Wood is a winning phrase.

Millie Richardson, Penpinelli, Gwent, Wales

It was used during the days of sail when fate was tempted to conjure up a wind when becalmed or such.

John Bonehill, San Francisco, USA

The phrase comes from when in back in the days people belived that bad spirits lived in the wood so they would knock or tap so that the bad spirits wouldn't hear their hopes

Jayden Calderbank, Upper Norwood London

When ever I have heard the phrase or used the phrase myself it has been order to not tempt fate. "I hope the plane doesn't crash" "Touch wood!" and it's been explained to me in the local pub to mean "Touch wood and an acorn won't fall" I don't know much more than that!

Justin Cliffe, Oswestry, Shropshire England

The phrase dates back to Chaucer's time and beyond when "summoners and pardoners" sold relics which were highly questionably parts of the True Cross (or the little toe of St James).
The relic (in this case, the piece of wood supposedly part of the Cross) was carried in one's pocket and at the hearing of plague, death or other misfortune would be touched for protection and safety because of the belief that the Lord Jesus had defeated the powers of darkness by his death on the Cross.

Paul Hockley, WARRINGTON UK

It comes from the lumberjacks in wooded areas who would "hug" a tree for a safe felling of the tree. Explaining the phrase "touch wood" (the clever woman on channel4's countdown explained this on an episode once.)

kim, dumfries scotland

Sailors would tap their foot on the deck of the ship

Michaela, Bath UK

Perhaps a term used by coal miners? They would knock on each wooden roof support, as they passed it, to test that it was not rotten.